ROME—An arts center built by the emperor Hadrian in A.D. 123 was found beneath Rome’s Piazza Venezia during excavations for a new subway line. Nobles came to the three brick-walled halls with terraced marble seating to hear poetry, speeches, and philosophy. After the collapse of the empire, the space was used for smelting metals until an earthquake in 848 brought down the roof. In the sixteenth century, a hospital was built on the site. The area had been slated for two rail stations.